The proposal that Kaposi's sarcoma undergoes cellular transformation from early to late stages was studied with argyrophilic nucleolar organizer regions, proliferating cell nuclear antigen/cyclin, and immunoperoxidase staining for factor VIII-related antigen. Staining of argyrophilic nucleolar organizer regions was significantly increased in the plaque/nodular stage compared to the patch stage. The endothelial-selective marker factor VIII-related antigen stained more intensely in patch stage lesions. This was inversely correlated with staining of argyrophilic nucleolar organizer regions. Proliferating cell nuclear antigen/cyclin staining did not correlate with tumor stage or with factor VIII-related antigen. The changes in argyrophilic nucleolar organizer regions and factor VIII-related antigen staining are evidence for cellular transformation in Kaposi's sarcoma.